Fuel injection systems with which fuel injection into a combustion chamber of an internal combustion engine is performed have long been known. Injection systems of this kind comprise at least one injection valve (injector) and at least one feedforward and feedback control unit, connected to the injection valve, for controlling the injection process. Here, the injection valve has a space from which fuel can be injected into the combustion chamber through an injection opening. The opening and closing of the injection opening is performed by means of a closure element (nozzle needle), which can be actuated (moved) by an actuator. The space is supplied with fuel via a high-pressure reservoir and a fuel line.
The actuator is an element for moving the closure element. Thus, an injection process is controlled with the aid of the actuator. At the same time, the actuator is not in direct drive connection with the closure element but actuates a servo valve in order to discharge fuel under high pressure from a servo valve space and, in this way, to bring about actuation of the closure element and to open the associated injection opening.
The actuator is a piezoelectric actuator which expands (increases in length) by virtue of the piezoelectric effect when supplied with electrical energy and in this way raises the servo valve from its seat in order thereby to actuate the closure element.
In order to carry out a pressure reduction in the pressure reservoir (rail pressure reduction) in such fuel injection systems with piezoelectric servo injection valves, special pressure control valves (PLV, PCV, PDV) are used in prior art systems. These additional valves increase the costs of the overall system. In another procedure, a pilot-controlled servo valve is used in reducing the pressure in the pressure reservoir. With a pilot-controlled servo valve of this kind, however, it is not always possible to reduce the pressure in the pressure reservoir in the desired manner, and therefore it is nevertheless necessary to provide additional valves for pressure reduction, depending on customer requirements and the injection valve design.